1. Field of the Invention
The invention relates generally to serial attached SCSI (“SAS”) domains and more specifically to methods and structures for more easily detecting SAS link errors in a SAS expander while minimizing impact on the SAS initiators and on SAS link bandwidth utilization.
2. Discussion of Related Art
Small Computer Systems Interface (“SCSI”) is a set of American National Standards Institute (“ANSI”) standard electronic interface specification that allow, for example, computers to communicate with peripheral hardware. Common SCSI compatible peripheral devices may include: disk drives, tape drives, CD drives (“CD-ROM”, “CD-RW”, etc), DVD drives, printers and scanners. SCSI as originally created included both a command/response data structure specification and an interface and protocol standard for a parallel bus structure for attachment of devices. SCSI has evolved from exclusively parallel interfaces to include both parallel and serial interfaces. “SCSI” is now generally understood as referring either to the communication transport media (parallel bus structures and various serial transports) or to a plurality of primary commands common to most devices and command sets to meet the needs of specific device types as well as a variety of interface standards and protocols.
The collection of primary commands and other command sets may be used with SCSI parallel interfaces as well as with serial interfaces. The serial interface transport media standards that support SCSI command processing include: Fibre Channel, Serial Bus Protocol (used with the Institute of Electrical and Electronics Engineers 1394 FireWire physical protocol; “IEEE 1394”) and the Serial Storage Protocol (SSP).
SCSI interface transports and commands are also used to interconnect networks of storage devices with processing devices. For example, serial SCSI transport media and protocols such as Serial Attached SCSI (“SAS”) and Serial Advanced Technology Attachment (“SATA”) may be used in such networks. These applications are often referred to as storage networks. Those skilled in the art are familiar with SAS and SATA standards as well as other SCSI related specifications and standards. Information about such interfaces and commands is generally obtainable at the website http://www.t10.org.
Such SCSI storage networks are often used in large storage systems having a plurality of disk drives to store data for organizations and/or businesses. The network architecture allows storage devices to be physically dispersed in an enterprise while continuing to directly support SCSI commands directly. This architecture allows for distribution of the storage components in an enterprise without the need for added overhead in converting storage requests from SCSI commands into other network commands and then back into lower level SCSI storage related commands.
A SAS network typically comprises one or more SAS initiators coupled to one or more SAS targets often via one or more SAS expanders. In general, as is common in all SCSI communications, SAS initiators initiate communications with SAS targets. The expanders expand the number of ports of a SAS network domain used to interconnect SAS initiators and SAS targets (collectively referred to as SAS devices or SAS device controllers).
In general, a SAS initiator directs information to a SAS target device through ports of one or more SAS expanders in the SAS domain. A “port” in SAS terminology is a logical concept. A port may comprise one or more physical links in a SAS domain. Such physical links are often referred to as PHYs in the terminology of SAS domains. A port may use a single PHY or, if the port is configured as a wide port, may use multiple PHYs logically grouped to provide higher bandwidth.
Link errors during such transmissions may result in bad frame transmissions, lost frames or primitives, etc. To avoid substantial loss of data integrity due to such errors, SAS initiators at present periodically scan or poll the ports of a SAS expander to attempt to quickly detect when an error has occurred. Any such scanning or polling of expander links by a SAS initiator uses valuable communication bandwidth on the SAS communication links (i.e., on one or more PHYs in the SAS domain coupling SAS expanders to the SAS initiator). In addition, in hopes of quickly detecting occurrence of such errors, the SAS initiator may frequently perform such scans to detect the problem quickly after initial occurrence of the error. Such frequent scanning or polling by a SAS initiator further exacerbates the problem of abusing available bandwidth.
It is a particular problem to detect link level errors in operation of SAS expanders in an efficient manner reducing overhead consumption of communication bandwidth.
It is therefore evident from the above discussion that a need exists for improved solutions in a SAS domain to quickly identify physical link errors and to do so without significant impact on the available bandwidth of the SAS domain.